Everybody has their own approach. For folks who go further than choosing based on utility or intended use, the main factors usually include timeframe, region,and style. The typical timeframes are “early” meaning pre-Revolutionary War through the war years, “Golden Age” encompassing post war through maybe 1812, late flint/Southern mountain rifle/early fur trade, and percussion plains rifles. Regions are roughly Virginia and Pennsylvania and unknown for many early rifles, and so on and so on.
Many folks study books and originals in museums and find something that really appeals and then go about making one like that.
All depends on how deep you are into it. Deeper is not better, it’s just deeper. Choosing a gun based on what’s available in kit form from a respected supplier seems a common approach as well. Choose your poison and enjoy the ride.